1. Field of the Invention
This invention relates to bioherbicides for controlling pest plants such as weeds and more particularly, to a method for biological control of a variety of pest plants including crabgrass using two species of a fungus selected from the genus Curvularia. Typically, a selected concentration of conidia in a liquid surfactant such as nonoxynol is applied to the pest plants to produce typical lesions which kill or suppress, and thus control, the plants. Alternatively, the conidia may be applied to the plants using an organosilicone surfactant such as Silwet L-77 (trademark), which has been found to enhance the pathogenic action of Curvularia. Granular preparations of the fungus are suitable for preemergence or postemergence application. Curvularia may be applied to crops in combination with the fungus Alternaria cassiae to control a broader range of pest plants than is achieved using either fungus alone.
Weeds present a tremendous problem to farmers throughout the world, causing an estimated 10-12% loss of value for agricultural products in the United States, the most recent estimate being $20 billion annually (McWhorter, C. G. [1984] Weed Science, 32:850-855). Chemical pesticides are commonly used to control pest grasses in agricultural crops, but concern over environmental damage caused by these pesticides has recently elicited societal pressures to replace the chemical pesticides with alternative control methods. One area of active research in this area involves the use of plant pathogens, including both bacteria and fungi, to attack and kill pest plants in agricultural crops.
A major constraint to commercial development of a plant pathogan as a biological herbicide is selectivity. A pathogen that controls only one weed species in one type of crop does not have the same market potential as a pathogen that controls several important weed species in different types of crops. It has surprisingly been found that two fungus species of the genus Curvularia are effective in controlling multiple varieties of pest grasses in several different types of important agricultural crops. In addition to controlling crabgrass species, the fungi utilized in this invention are effective in controlling barnyard grass, green foxtail and other susceptible pest grasses, but do not harm sugar cane or broad-leafed crop plants such as soybean, cotton and peanuts. Since Bermuda grass, St. Augustine grass, Centipede grass and Zoysia grass are also unharmed by the fungus, Curvularia may also be used to control weed growth in residential and commercial yards. In addition to the pest grasses mentioned above, sicklepod may also be controlled by using the fungus Alternaria cassiae Curvularia in combination with Curvularia.
2. Description of the Prior Art
Several methods are known in the art for using biological organisms to control weeds and other pest plants. As disclosed in U.S. Pat. No. 3,999,973, to Daniel, et al, the anthracnose fungus Coltetotrichum gloeosporioides has been used to control the weed northern jointvetch and another strain of this fungus has been used to control winged waterprimrose. Colletotrichum malvarum has been used to control prickly sida. These three pathogens have been combined to control all three target weeds at once. In other experimental work the fungus Alternaria macrospora has been used to control spurred anoda (Anoda cristata), Weed Science, H. L. Walker, 1981, Vol. 29, pp.505-507.
My U.S. Pat. No. 4,390,360, dated Jun. 28, 1983, describes "Control of Sicklepod, Showy Crotalaria and Coffee Senna With A Fungal Pathogen" using a specific host strain of the fungus Alternaria cassiae to produce typical weed lesions which kill or suppress the respective weeds. My U.S. Pat. No. 4,419,120, dated Dec. 6, 1983, discloses "Control of Prickly Sida, Velvetleaf and Spurred Anoda With Fungal Pathogens" using a specific host strain of the fungus Fusarium lateritium to kill or suppress the respective weeds. U.S. Pat. No. 4,715,881, dated Dec. 29, 1987, to Andersen, et al, details "Control of Eastern Black Nightshade With a Fungal Pathogen" using a strain of Colletotrichum coccodes which is selectively pathogenic toward eastern black nightshade (Solanum ptycanthum). My U.S. Pat. No. 4,718,935, dated Jan. 12, 1988 and U.S. Pat. No. 4,767,441, dated Aug. 30, 1988, describe a "Method For The Preparation of Mycoherbicide-Containing Pellets" characterized by alginate gel pellets containing living fungus capable of producing conidia when exposed to sufficient light and moisture. U.S. Pat. No. 4,724,147, dated Feb. 9, 1988, to James J. Marois, et al, and U.S. Pat. No. 4,818,530, dated Apr. 4, 1989, also to James J. Marois, et al, both detail the "Preparation of Pellets Containing Fungi for Control of Soilborne Diseases", in which fungi are first selected and grown for a time sufficient to produce inoculum. The fungal propagules are harvested, homogenized and diluted with sodium alginate solution. Pelletization is then accomplished by dropwise addition of the fungal propagule-alginate mixture into a solution of calcium chloride or calcium gluconate. The resulting alginate gel pellets containing living fungi can then be dried and used to inoculate agricultural fields infested with soilborne plant diseases. U.S. Pat. No. 5,192,541, dated Mar. 9, 1993, to Steven D. Savage, et al, describes "Weed-Killing Xanthomonas campestris", in which novel microorganisms useful in controlling unwanted grasses and other weeds are discovered through a unique process which involves isolating plant pathogens from asymptomatic plants.